Yoma 1:2-3

For the entire week leading up to Yom Kippur, the Kohein Gadol sprinkles the blood of the twice-daily Tamid offerings, he burns the incense, changes the lights of the Menorah, and offers the head and leg of the Tamid. Any other day of the year, he had right of first refusal in any part of the Temple service. The Kohein Gadol has first rights to offer any portion (not having to draw lots like other kohanim) and to take any portion (even if he did not participate in offering a sacrifice).

Yoma 1:3

Each day of that week, the Kohein Gadol was brought Sages from the Sanhedrin, who would read the order of the Yom Kippur service to him. They would instruct him to read it aloud himself to ensure that he was proficient in the service. On the morning of the day before Yom Kippur, they stood the Kohein Gadol in the Eastern Gate and passed oxen, rams and sheep before him to familiarize him with the order of the Yom Kippur service.